1. Field of the Invention
The present invention relates to a method and device useful for the measurement of high reflectivity autostigmatic optical elements and systems. More particularly, the invention relates to the use of a thin, partially transmissive optical element which is placed in the measurement leg of a spherical wavefront Fizeau interferometer to produce high contrast two-beam interference fringes.
2. The Prior Art
The development of the laser and advances in vacuum coating technology have greatly expanded the utility of classical interferometers. The Fizeau interferometer, in particular, has become an extremely convenient and flexible instrument for a wide variety of optical metrology applications. Nevertheless, a conspicuous shortcoming of the laser Fizeau interferometer has been in the testing of high reflectivity autostigmatic elements and systems. A multiple beam spherical wavefront interferometer is discussed in detail in an article by Heintze et al in Applied Optics, Vol. 6, p. 1924, November 1967. The authors propose a Fizeau multiple beam spherical wavefront interferometer with a field lens at the center of curvature of the reference surface. The major difficulties with the interferometer discussed by Heintze et al are: (1) the partially transmissive coating on the spherical reference surface must be selected to match closely the reflectivity of the test article to achieve useful contrast. Therefore, a number of these expensive elements is required to handle a range of test article reflectivities, and (2) a field lens which matches each test article radius is required.
Other types of interferometry have been used to test high reflectivity autostigmatic optical elements and systems. For example, scatter plate interferometers and shearing type interferometers are two prominent means. However, these interferometers are not only difficult to use and align, but they are also considerably less versatile than the Fizeua interferometer.
While these prior-art techniques are usful for some applications, they cannot be used in many optical metrology applications because of the specific, close match of the elements required to carry out the measurement to the parameters of the test article. To this end, a method and device are required for testing high reflectivity autostigmatic optical elements and systems which do not require the specific, close match of expensive elements to the parameters of the test article.